You may find that after you add a disk to Windows Server 2012 it doesn’t show up as a drive. This is because disk management has changed a little in Windows Server 2012 and is now called File and Storage Services. File and Storage Services can still be found in the same place that Disk Manager, but it’s less obvious how to add a disk in Server 2012.
To add a disk to Server 2012, in the Server Manager, go to File and Storage Services and click on Disks. Right click the disk you have just added and click Bring Online, clicking Yes when prompted. Once the disk is online, right click the disk again and click New Volume, following the prompts to create a new volume.
Once completed, you should be able to see the disk you added.